PROJECT SUMMARY (PILOT PROJECT 2) Breast cancer (BC) is the second leading cause of cancer death in women. About 75% of all BCs express estrogen receptor-? (ER). While endocrine therapies targeting ER or the synthesis of its ligand estrogen are effective, resistance to these drugs is quite common and presents a major clinical challenge in the treatment of ER-positive (ER+) BC. About 50% of metastatic ER+ disease present with de novo endocrine resistance, and most of remaining patients eventually acquire endocrine resistance. Even in the adjuvant setting, about 30% of patients treated with endocrine therapy eventually develop recurrence suggesting endocrine-resistant residual disease, which often progresses to distant organs as metastatic disease. Therefore, prevention and effective treatment of endocrine resistance is of paramount significance in improving outcomes as well as survival in ER+ BC patients. Resistance to endocrine therapies targeting ER is caused by many alternate and escape pathways. However, ER remains an essential molecular target even in the resistant setting as suggested by several lines of clinical and preclinical evidence. These studies suggest that endocrine-resistant tumors have reprogramming of ER transcriptional activity, which is no longer sensitive to existing endocrine therapy. In this proposal, we aim to investigate FOXA1, a master regulator of ER reprogramming in endocrine resistance, as a drug target by combining unique expertise of drug discovery-development groups at the University of Houston College of Pharmacy and DLDCCC. In this collaborative research proposal, we propose to initiate a drug discovery-development program by (1) utilizing DNA-Encoded Chemistry Technology!(DEC-Tec) compound libraries as a primary high throughput binding assay for drug discovery (DLDCCC), (2) applying medicinal chemistry efforts to synthesize the hits as well as structure activity relationship (SAR) (UHCOP), (3) developing and implementing a cell-free functional assay as a secondary assay to test and validate the hits from the primary screen (UHCOP), and (4) employing two target-specific cell-based assays as tertiary assays for preliminary hit-to-lead identification in drug discovery-development process (UHCOP and DLDCCC). In summary, our efforts to discover novel drugs may have a potential for a major clinical impact in overcoming drug resistance in ER+ BC. Because of the role of FOXA1 in many primary and metastatic tumors, our research will have broad clinical significance in the treatment of cancer. We also expect that our proposed research will advance the unique cancer drug discovery and development collaborative efforts between DLDCCC and UHCOP and serve as a platform for creating cancer research training opportunities for underrepresented (UR) trainees at UH. Despite similar prevalence in ER+ breast cancer, African American women have higher mortality likely due to endocrine resistance. Therefore, drug discovery-development research to overcome endocrine resistance in ER+ BC will be of high interest to UR trainees.